The invention relates to a filter device with a chamber with an inlet for the material to be filtered, a filtrate outlet and a residue outlet serving to discharge the retained residues, and a helicoidally conformed support arranged in the chamber, also a filter retained by said support.
A known filter device of this type exhibits a cylindrical chamber, in the axis of which a hollow rod is arranged, the cavity of which is connected to the outlet, and which is provided with holes distributed along its length. A support rod of solid circular cross-section, which revolves helicoidally around the hollow rod, is also present. An interstice is also present between consecutive convolutions of the helicoidally conformed support rod. Furthermore, an interstice is also present between the hollow rod located in the chamber axis and the convolutions of the helicoidally conformed support rod. The filter is constituted by a filter cloth. Said cloth originally exhibited the shape of a cylindrical tube. Said tube was then fitted over the support rod in the direction of the chamber axis. The tube was then constricted by a cord running helicoidally between the convolutions of the support rod so that it is drawn between the consecutive convolutions of the support rod against the hollow rod extending along the chamber axis. The filter cloth then forms a surface which resembles the external surface of a screw. But since the filter cloth is constricted between the convolutions of the support rod to a diameter which is considerably smaller than the diameter of the hose originally constituted by the filter cloth, folds extending more or less radially are produced in the filter cloth. The size of said folds increases from the circumference towards the hollow rod.
During operation the suspension to be filtered is introduced into the chamber. The liquid component of the suspension can then pass through the surface filter and the holes in the hollow rod into its cavity and through the latter to the filtrate outlet. The solids present in the suspension are retained on the surface of the filter cloth and must be removed from time to time.
However, said removal of the solids deposited upon the surface of the filter cloth is comparatively difficult because of the folds referred to. Said folds also have the result that an uneven distribution of the separated solids on the surface of the filter cloth is obtained. This increases the clogging frequency of the filter cloth, so that the relatively difficult removal of the solids is moreover necessary after comparatively short operating periods each time.
In many cases, particularly where the filtration is performed in order to recover the solid residue, it would be desirable to generate an overpressure in the chamber during the filtration by means of a pressurized gas. This however is practically impossible with the filter previously known because the screw convolutions formed by the filter otherwise become considerably compressed.